Optical glass or an optical transparent resin is used as a material for an optical device used in an optical system of various kinds of cameras, such as a camera, a one-time use camera and a video camera. Optical glass is excellent in heat resistance, transparency, dimensional stability, chemical resistance and the like, and includes various materials having various refractive indices and Abbe numbers, but has problems of high material cost, poor molding workability and low productivity. In particular, formation of an aspherical lens used for aberration correction requires extremely high technique and high cost, which result in a significant barrier on practical use.
On the other hand, an optical lens formed of an optical transparent resin, particularly a thermoplastic transparent resin, has such advantages that the lens can be mass-produced by injection molding and also can be formed easily into an aspherical lens, and thus the lens is currently applied to a lens for a camera. Examples of the resin include polycarbonate containing bisphenol A as a constitutional unit, polymethyl methacrylate and amorphous polyolefin.
In general, aberration of an optical system of a camera is corrected by the combination use of plural of convex lens and a concave lens. Specifically, chromatic aberration formed by a convex lens and a chromatic aberration of a concave lens that is opposite to that of the convex lens are combined to negate the chromatic aberration. In this case, the concave lens is required to have high dispersion (i.e., a low Abbe number).
As for the dispersion (Abbe number) of the aforementioned optical thermoplastic resins, polycarbonate containing bisphenol A as a constitutional unit has a refractive index of approximately 1.59 and an Abbe number of approximately 32, polymethyl methacrylate has a refractive index of approximately 1.49 and an Abbe number of approximately 58, and amorphous polyolefin has a refractive index of approximately 1.54 and an Abbe number of approximately 56. Only polycarbonate may be used as a concave lens for aberration correction, but its Abbe number of 32 cannot be considered as sufficiently high dispersion.
Patent Document 1 discloses polycarbonate containing 3,9-bis(1,1-dimethyl-2-hydroxyethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane as a constitutional unit, but the Abbe number thereof is larger than that of polycarbonate containing bisphenol A as a constitutional unit.
Patent Document 2 discloses a polyester resin composition formed by copolymerization of a fluorene dihydroxy compound having a refractive index of 1.66 and an Abbe number of approximately 20 as a resin for use as a concave lens for aberration correction. The resin has a sufficiently large dispersion (i.e., a low Abbe number) but has the following defects as a resin for an optical lens. That is, the resin is formed by copolymerization of a large amount of a fluorene dihydroxy compound, which is bulky and rigid, and thus the resin has a considerably high melt viscosity and is poor in moldability. As a measure for improving the moldability, the melt viscosity upon molding may be lowered, i.e., the molding temperature may be increased, but such problems may occur thereby that coloration upon molding is intensified, and the mold is contaminated with thermal decomposition products. Such a measure is also considered that the melt viscosity is lowered by decreasing the polymerization degree, but the case involves such problems that the content of low molecular weight components in the resin is relatively increased to contaminate the mold with the low molecular weight components or decomposition products of the low molecular weight components. Accordingly, there has been no proposal of an optical lens formed of a thermoplastic resin that has both excellent optical characteristics (i.e., a high refractive index and a low Abbe number) and practically sufficient moldability.
Patent Document 3, which belongs a technical field that is different from the present invention, discloses a lens sheet for a liquid crystal display device, containing an optical device formed on a surface of a transparent substrate containing a polyester resin that is constituted by a diol unit, in which from 1 to 80% by mol of the diol unit has a cyclic acetal skeleton, and a dicarboxylic acid unit.